Iran's supreme leader said late on Monday that his country would need to significantly increase its capacity to enrich uranium if it was to meet its long-term energy needs, in an unusually detailed speech highlighting the obstacles to a deal on its nuclear programme.

Ayatollah Ali Khamenei conceded that Iran would not need to immediately increase its capacity but made clear that his government sought the right to carry out industrial-scale enrichment in order to be self-sufficient in nuclear fuel for its research reactors and a Russian-built power station at Bushehr.

Enrichment capacity is the main obstacle to a comprehensive agreement between Iran and six major powers taking part in talks in Vienna. Western negotiators want Iran to be restricted to a research-scale capability to minimise the risk it could build a nuclear weapon at short notice but by publicly stating Iran's position, Khamenei could have made it harder for his negotiators to compromise.

"It is very unhelpful to say that in public," said Mark Fitzpatrick, a former US state department non-proliferation expert now at the International Institute for Strategic Studies in London. "It's nothing different from what the Iranian negotiators have said privately, but to say it publicly boxes in the negotiators and makes it harder to climb down."

"The silver lining is that he says Iran doesn't need this capacity immediately but that doesn't help much. The six powers will argue Iran doesn't need industrial-scale enrichment. It would be terribly unsafe for Iran to use domestically-fabricated fuel in Bushehr. Khamenei has just made it harder to get a deal."

Iran currently has 19,000 centrifuges installed at its enrichment plants in Natanz and Fordow, but only about 10,000 are operational. The negotiating states in Vienna – the US, UK, France, Germany, China and Russia – have hitherto insisted on fewer than 10,000 in total so it would take Iran more than six months to change its civilian programme to a military one and build a warhead.

In the remarks made to senior Iranian officials including the country's president, Hassan Rouhani, Khamenei did not talk about Iran's needs in terms of numbers of centrifuges but overall enrichment capacity, expressed in a specialist term, "separative work units" or SWU, measured in kg per year.

"On the issue of enrichment capacity, their [the west's] aim is make Iran accept 10,000 SWU," Khamenei said. "Our officials say we need 190,000 SWU. We might not need this [capacity] this year or in the next two or five years but this is our absolute need and we need to meet this need."

The old centrifuges Iran is currently using, known as IR-1s, are currently running on a capacity of below one SWU a year, so Khamenei's target would require more than 200,000 of them, over 20 times the proposed limit. However, the head of Iran's atomic energy organisation, Ali Akbar Salehi, pointed out that Iran was working on much more sophisticated centrifuges, including IR-6s which he claimed would have a capacity of 24 SWU, comparable with modern western machines. In that case, Iran would only need 7,000 of them.

However, nuclear experts say that Salehi's ambitions sound far-fetched. Iranian technicians have been struggling for years to perfect a second generation machine, the IR-2m, with a theoretical capacity below seven SWU. Furthermore, if such powerful centrifuges were used, the west would insist they be used in far smaller numbers.

"The fight over centrifuge capacity is mirroring the full 35-year clash between the US and Iran: The US wants Iran to back down and accept American preeminence," Trita Parsi, the head of the National Iranian American Council, an advocacy group in Washington, said.

"Iran wants to reclaim its dignity by standing firm in face of what it sees as bullying. In that sense, the centrifuge numbers have taken on a much deeper and more worrisome meaning."

However, Parsi pointed out there could be some room for flexibility reflected in Khamenei's remarks. "It's interesting that he refers to what the experts say, rather than coming down hard with his own firm stance. Also, he keeps the timeline undetermined, which can open the way for compromise."